The North Koreans have said that the 'mushroom cloud' which people feared was a nuclear test, was actually earthworks. They were destroying a mountain as part of a dam project.
There has been much speculation over the past few days about this mushroom cloud - especially given that it was on the Chinese border and the inherent risks of an open air test to the Koreans themselves. I'm inclined not to think it was nuclear in nature, as no fallout was detected. However, whether this is the correct explanation remains to be seen.

Mushroom clouds form due to vast convection currents, and though they are linked to nuclear explosions (due to the vast temperature differences) they can be associated with any event which sets up a suitable current.